The answer can be seen in the black box mounted roughly in line with the second hinge from the bottom. Of the ten hinges used on this pair of doors, nine are spring hinges (AKA door-slammers), however the one different hinge is in fact an electric power transfer (EPT) hinge. The black box on the wall is a credential reader (most likely prox card or key fob) that is used for access to the apartments. The EPT hinge connects to the exit device mounted on the door on the right (RHR swing for all you door geeks). When the appropriate credential is presented, the EPT hinge powers the exit device, and either retracts the bolts or unlocks the lever allowing residents to enter. So there you have it! Unusual doors with some interesting hardware stuff going on as well!